Emulsifier-stabilized aqueous dispersions of vinyl acetate-ethylene copolymers are used in a wide variety of applications, for example as binders in adhesive compositions and in coating compositions. These are high volume applications, and therefore large amounts of such dispersions have to be produced on an industrial scale. Shortening of cycle times of the polymerization of emulsifier-stabilized vinyl acetate-ethylene copolymer dispersion would result in a considerable economic advantage. At the same time, a key issue for obtaining high quality binders for such applications is the need to produce the dispersed vinyl acetate-ethylene copolymer particles with a fine particle size.
It is known in the art to use external cooling loops in emulsion polymerization for enhancing cooling capacity to achieve a shorter reaction time. For the shortening of polymerization time it is disclosed in U.S. Pat. No. 6,660,814 B1 to use a polymerization reactor combined with an external cooling unit, which is equipped with a low-shear pump and a heat exchanger having a laminar flow profile. EP 2 106 849 A1 teaches a process for emulsion polymerization. To improve heat removal, the polymerization reactor is combined with an external cooling loop, which is equipped with a low-shear pump and a plate and frame heat exchanger. To prevent coagulation of the polymerisation mixture, the plate and frame heat exchanger has a gap width of 6 to 18 mm. U.S. Pat. No. 6,320,000 B1 concerns an emulsion polymerization in the presence of emulsifiers. The polymerization takes place in a reactor equipped with an external cooling circuit, which includes a pump and a heat exchanger. To improve cooling capacity and to prevent gel formation, the polymerization mixture is pumped through the heat exchanger and returned to the polymerization reactor. The object of US 2005/0261423 A1 was a polymerization process providing shorter reaction time. The goal was said to be achieved with a process wherein the polymerization mixture is transferred to an external circuit which leads from and back to the polymerization reactor and which comprises a low-shear pump and a heat exchanger. US 2011/0112218 A1 describes a process for the production of a protective colloid-stabilized aqueous polymer dispersion. To improve heat dissipation and thereby shorten reaction time, an external loop is installed which includes a pump and a heat exchanger. The process uses a specific type of heat exchanger, i.e., a static mixer heat exchanger.
One problem with aqueous emulsifier-stabilized vinyl acetate-ethylene copolymer dispersions with fine particle size is their typically poor shear stability. This problem has traditionally prevented production of such dispersions in polymerization reactors employing external heat exchange circuits, which typically produce high shear. In order to make vinyl acetate-ethylene copolymer dispersions in reactors with external heat exchangers, some way must be found to increase the shear stability of the dispersions while nonetheless maintaining fine particle size, as well as good scrub resistance in paints and other coatings made from the dispersions.
US 2008/0262104 A1 discloses polyvinyl acetate dispersions having high shear stability with 0.05 to 5% by weight of (meth)acrylic acid units in the vinyl acetate copolymer. US 2007/0244238 A1 describes polymer dispersions said to have very good shear stability. These polymer dispersions comprise two copolymers with a Tg difference of at least 10° C., stabilized with a nonionic emulsifier comprising at least one aryl-substituted phenolalkyleneoxy ether.
EP 1 300 448 A2 and EP 1 801 168 disclose an aqueous vinyl acetate-ethylene dispersion used as a binder in coating compositions having a high pigment volume concentration. To improve scrub resistance, use of a combination of a phosphate-functional emulsifier and a protective colloid for stabilization during polymerization is recommended.
Despite the foregoing efforts, it has remained difficult to produce vinyl acetate ethylene copolymer dispersions having fine particle size in equipment employing an external heat exchanger. Thus, advances in this area would be a welcome addition in the industry.